Low and mean density polyethylenes (values generally lower than 0.945) obtained by low pressure polymerization processes in which ethylene is polymerized in admixture with an alpha-olefin, in particular propylene or butene, in the presence of Ziegler-type catalysts or of catalysts based on silica-supported oxides, are already known.
The amount of polymerized alpha-olefin contained in those polymers generally ranges from 1 to 15% by moles. The ratio between the molar percentage of alpha-olefin and the polymer density generally is not lower than 5-6 for density values equal to about 0.910.
A high content of alpha-olefin in the polyethylene chain on one side contributes to a lowering of the polymer density and, on the other side, adversely effects all the mechanical properties of said polymer.
Thus, there has been a need for polymers which, for a given density, have a minimum content of alpha-olefins.
Copolymers endowed with said characteristics (low density and simultaneously low content of alpha-olefin) are described in Canadian Patent No. 849,081. Those copolymers, however, are characterized by a relatively low melting point, comprised between about 122.degree. and 102.degree. C. for density values decreasing from about 0.930 to 0.910 g/cc. In the field of the copolymers which, for a given density, have a low content of alpha-olefin, it is desirable for insuring good mechanical properties, that the polymer melting point be as high as possible.